Site Formation Processes - How Did That Archaeological Site Get There?

Why is an Archaeological Site Like a Palimpsest?

Site Formation Processes at Work
Site Formation Processes at Work. Tobin

Site Formation Processes—or more simply formation processes—refers to the events that created and affected an archaeological site before, during and after its occupation. To gain the best possible understanding of an archaeological site, researchers collect evidence of the natural and cultural events that happened there. A good metaphor for an archaeological site is a palimpsest, a medieval manuscript that has been written on, erased, and written over, again and again and again.

Archaeological sites are the remains of human behaviors, stone tools, house foundations, and garbage piles, left behind after the occupants leave. However, each site was created in a specific environment--lakeshore, mountainside, cave, grassy plain. Each site was used and modified by the occupants--fires, houses, roads, cemeteries were built; farm fields were manured and plowed; feasts were held. Each site was eventually abandoned--as a result of climate change, flooding, disease. By the time the archaeologist arrives, the sites have lain abandoned for years or millennia, exposed to weather, animal burrowing, and human borrowing of the materials left behind. Site formation processes include all of that and quite a bit more.

Natural Transforms

As you might imagine, the nature and intensity of events that occurred at a site are highly variable. Archaeologist Michael B. Schiffer was the first to clearly articulate the concept in the 1980s, and he broadly divided site formations into the two major categories at work, natural and cultural transforms.

Natural transforms are ongoing, and can be assigned to one of several broad categories; cultural ones can end, at abandonment or burial, but are infinite or close to it in their variety.

Changes to a site caused by nature (Schiffer abbreviated them as N-Transforms) depend on the age of the site, the local climate (past and present), the location and setting, and the type and complexity of occupation.

At prehistoric hunter-gatherer occupations, nature is the primary complicating element: mobile hunter-gatherers modify less of their local environment than do villagers or city dwellers.

Types of Natural Transforms

  • Pedogenesis, or the modification of mineral soils to incorporate organic elements, is an ongoing natural process. Soils constantly form and reform on exposed natural sediments, on human-made deposits, or on previously formed soils. Pedogenesis causes changes in color, texture, composition, and structure: in some cases, it creates immensely fertile soils such as terra preta, and Roman and medieval urban dark earth.
  • Bioturbation, disturbance by plant, animal and insect life, is particularly difficult to account for, as shown by a number of experimental studies, most memorably with Barbara Bocek's study of pocket gophers. She discovered that pocket gophers can repopulate the artifacts in a 1x2 meter pit backfilled by clean sand in the space of seven years.
  • Site burial, the burial of a site by any number of natural forces, can have a positive effect on site preservation. Only a handful of cases are as well-preserved as the Roman site Pompeii: the Makah village of Ozette in Washington state in the US was buried by a mudflow about 1500 AD; the Maya site Joya de Ceren in El Salvador by ash deposits about 595 AD. More commonly, the flow of high- or low-energy water sources, lakes, rivers, streams, washes, disturb and/or bury archaeological sites.
  • Chemical modifications are also a factor in site preservation. These include cementation of deposits by carbonate from groundwater, or iron precipitation/dissolution or diagenetic destruction of bone and organic materials; and the creation of secondary materials such as phosphates, carbonates, sulfates, and nitrates.

Anthropogenic or Cultural Transforms

Cultural transforms (C-Transforms) are far more complicated than natural ones, because they consist of a potentially infinite variety of activities. People build up (walls, plazas, kilns), dig down (trenches, wells, privies), set fires, plow and manure fields, and, worst of all (from an archaeological point of view) clean up after themselves.

Investigating Site Formation

To get a handle on all of these natural and cultural activities in the past that have blurred the site, archaeologists rely on an ever-growing group of research tools: the primary one is geoarchaeology.

Geoarchaeology is a science allied with both physical geography and archaeology: it is concerned with understanding the physical setting of a site, including its position in the landscape, types of bedrock and quaternary deposits, and the types of soils and sediments within and outside of the site. Geoarchaeological techniques are often carried out with the aid of satellite and aerial photography, maps (topographic, geological, soil survey, historical), as well as the suite of geophysical techniques such as magnetometry.

Geoarchaeological Field Methods

In the field, the geoarchaeologist conducts systematic description of cross-sections and profiles, to reconstruct stratigraphic events, their vertical and lateral variations, in and outside of the context of archaeological remains. Sometimes, geoarchaeological field units are placed off-site, in locations where lithostratigraphic and pedological evidence can be collected.

The geoarchaeologist studies the site surroundings, description and stratigraphic correlation of the natural and cultural units, as well as sampling in the field for later micromorphological analysis and dating. Some studies collect blocks of intact soils, vertical and horizontal samples from their investigations, to take back to the laboratory where more controlled processing can be conducted than in the field.

Grain size analysis and more recently soil micromorphological techniques, including thin section analysis of undisturbed sediments, are conducted using a petrological microscope, scanning electron microscopy, x-ray analyses such as microprobe and x-ray diffraction, and Fourier Transform infrared (FTIR) spectrometry. Bulk chemical (organic matter, phosphate, trace elements) and physical (density, magnetic susceptibility) analyses are used to incorrporate or determine individual processes.

Some Recent Formation Process Studies

  • Restudy of Mesolithic sites in Sudan excavated in the 1940s was conducted using modern techniques. The 1940s archaeologists commented that aridity had affected the sites so badly that there was no evidence of hearths or buildings or even post-holes of buildings. The new study applied micromorphological techniques and they were able to discern evidence of all of these types of features at the sites (Salvatori and colleagues).
  • Deep-water shipwreck (defined as shipwrecks more than 60 meters deep) site formation processes have determined that the deposit of a shipwreck is a function of heading, speed, time, and water depth and can be predicted and measured using a set basic equations (Church).
  • Formation process studies at the 2nd century BC Sardinian site of Pauli Stincus revealed evidence of agricultural methods, including the use of a sod buster and slash and burn farming (Nicosia and colleagues).
  • The microenvironments of Neolithic lake dwellings in northern Greece were studied, revealing a previously unidentified response to rising and falling lake levels, with the residents building on platforms on stilts or directly on the ground as needed (Karkanas and colleagues).